Hydrated silicas are widely used as fillers, pigments, and reinforcing agents in formulating rubbrous materials. Hydrated silica is typically prepared by reacting an alkali metal silicate solution, for example, sodium silicate solution, with carbon dioxide to precipitate silica of the desired particle size. Sodium silicate solution is typically produced by one of two methods, namely, the fusion process and the hydrothermal process.
In the fusion process, silica sand (SiO.sub.2) and soda ash (Na.sub.2 CO.sub.3) are melted in a furnace at a temperature of about 1400.degree. C. The melt is then quenched in water to produce an aqueous sodium silicate solution. Although it is reported that sodium silicate having SiO.sub.2 :Na.sub.2 O weight ratios as high as 3.75:1 may be produced by the fusion process, it is generally not commercially practicable to produce sodium silicate by the fusion process having a SiO.sub.2 :Na.sub.2 O weight ratio greater than about 3.2:1, since such higher ratio sodium silicates have been found to be unstable under process conditions. Moreover, the fusion process is disadvantageous from energy consumption and maintenance standpoints.
In the hydrothermal process, silica sand (SiO.sub.2) is reacted with concentrated, i.e., about 50 percent, aqueous sodium hydroxide solution at a moderately elevated temperature to produce an aqueous sodium silicate solution. Typically, the sand and caustic solution are reacted in an autoclave at super-atmospheric pressure and a temperature of about 200.degree. C. The SiO.sub.2 :Na.sub.2 O weight ratio in sodium silicate solutions produced by the hydro-thermal process typically ranges from about 2:1 to a maximum of about 2.5:1. Thus, although the energy consumption of the hydrothermal process is considerably less than that of the fusion process, desirable high ratio sodium silicate, i.e., sodium silicate having a SiO.sub.2 :Na.sub.2 O weight ratio greater than about 2.5:1, cannot as a practical matter be produced.
It has now been found that the silica:alkali metal oxide weight ratio of alkali metal silicate, particularly a sodium silicate typically produced by the hydrothermal process, can be increased to a ratio at least as high and higher than that obtained by the fusion process with an overall reduction in energy consumption and maintenance costs.